There has been known a variable capacitor which has a structure that a thin-film lower electrode layer, a thin-film dielectric layer, and a thin-film upper electrode layer are deposited in this order on an electrically insulating supporting substrate, in which a dielectric material of barium strontium titanate ((BaxSr1-x)yTi1-yO3-z) (hereinafter also referred to as BST) is used for a material of the thin-film dielectric layer and a capacitance is changed by variation of dielectric constant induced by application of a predetermined bias potential between the upper electrode layer and the lower electrode layer (for example, refer to Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
Such capacitance variation of the variable capacitor occurs even in a high-frequency region and it is therefore available even in the high-frequency region. By using capacitance variation of the variable capacitor induced by a direct-current bias voltage in the high-frequency region, it is possible to obtain a useful electronic component which is capable of changing frequency characteristics.
For example, a voltage control type thin-film resonator in which the above-described variable capacitor and a thin-film inductor are incorporated, is capable of changing a resonant frequency by application of a direct-current bias voltage. Further, a voltage control type thin-film band-pass filter in which the variable capacitor or the voltage control type thin-film resonator, a thin-film inductor, and a thin-film capacitor are incorporated, is capable of changing a passband by application of a direct-current bias voltage. Further, the variable capacitor is also available in a voltage control type electronic component for microwaves (Japanese Unexamined Patent Publication JP-A 8-509103 (1996)).
The variable capacitor having the high dielectric constant thin film as described above is required to have high tunability and a high Q value as well as a low temperature coefficient, high power handling capability, high insulation resistance, a small distortion characteristic, no change with time, and the like. Note that the tunability represents a variable amount of the variable capacitor and is expressed in the expression: Tunability x=(C(0)−C(V)/C(0)×100(%) wherein C(0) represents a capacitance before voltage application (initial capacitance) and C(V) represents a capacitance after voltage application.
Further, there has been proposed a configuration that a variable capacitor includes a plurality of variable capacitance elements connected in series, each of which has the same configuration as each other, and a bias line for application of direct-current bias voltage is disposed on each of the variable capacitance elements. This makes it possible that the direct-current voltage is stably and evenly applied to the respective variable capacitance elements as well as that the high frequency voltage (high frequency signals) is divided for the respective variable capacitance elements. As a result, it is possible to provide a variable capacitor that a direct-current bias voltage-induced capacitance change is large while high frequency signal-induced capacitance change, noise, and nonlinear distortion can be reduced as well as that has excellent power handling capability (for example, refer to Japanese Unexamined Patent Publication JP-A 2004-165588).
The inventor has taken note of the high rate of capacitance change, a small distortion characteristic, high power handling capability, and the like, of the variable capacitor shown in JP-A 2004-165588, and in order to use these characteristics in an electronic component constituting a circuit for propagation of high frequency signals, he connected plural lines of the variable capacitors as mentioned above in parallel or in series, thus obtaining a variable capacitor array. In this case, a degree of freedom in selecting the initial capacitance C(0) becomes higher as a whole variable capacitor array, thereby allowing for a variable capacitor array having a desired value of the initial capacitance. For example, the initial capacitance C(0) can be higher in N lines of the variable capacitors connected in parallel. However, in the variable capacitor array having such a configuration, bias lines are provided so as to apply direct-current bias voltage to the respective variable capacitors separately, for example, so that the applied voltage (direct-current bias voltage which may be simply referred to as bias voltage) is equally applied to the respective lines of the variable capacitors connected in parallel, and even in this case, the tunability of the variable capacitor array as a whole is the same as that of the variable capacitor and thus not be able to be higher.
On the other hand, because the tunability is higher with a higher electric field intensity, the variable capacitor having such a thin film with high dielectric constant as being BST-made can meet a high tunability requirement by reducing a thickness of a dielectric thin film. However, the reduction in the film thickness deteriorates power handling capability, a leakage current characteristic, reliability, or the like, and therefore there is a limit to the control on the tunability through the film thickness.
In addition, the tunability depends on a material constituting the high-dielectric constant thin film, and the material is still being studied and no material has been known so far that can be mass-produced and attain tunability which meets demands of the market.
As described above, when plural lines of the variable capacitors having the same tunability are merely connected, there is a problem that the tunability of the variable capacitor array does not change and becomes constant, thus leading to a failure in meeting a still higher tunability requirement. Moreover, in the case where the variable capacitor is used in a mobile phone or the like, power consumption needs to be reduced, and besides, since the applicable direct-current bias voltage is limited, it is desirable to realize high tunability even when a voltage applied is low.